tetra voice coding ranko pinter simoco digital systems
TRANSCRIPT
Agenda
• Why code speech?
• Basic principles of TETRA voice
coding
• How was TETRA codec selected?
• Operational performance
• Future enhancements
Types of codec
• Waveform codec
•Transmitted bits represent the speech waveform
• Parametric codec•Transmitted bits drive a speech synthesiser
Parametric coding - Speech synthesis
Excitation generator
Synthesis filters
Synthetic speech
(Vocal tract)(Lungs, vocal chords)
Excitation generator
Synthetic speech
Pitch prediction filter
(Long term)
LPC synthesis filter
(Short term)
Perceptual error weighting filter
Speech synthesis
LPC - Linear Predictive Coding
Analysis-by-synthesis predictive coding
Speech input
Excitation generator
Synthesis filters
Perceptual error weighting
Error minimisation
Speech encoder
TETRA ACELP
Algebraic Code Excited Linear Predictive
Excitation generator
Pitch prediction filter
Algebraic codebook
Adaptive codebook
Audio processing (Tx)
Speech encoder
Homing function
Frame stealing
Encryption
Speech importance
Channel coding Digital output
Speech input
Audio processing (Rx)
Speech decoder
Homing function
Missing frame generation
Digital input
Speech output
Missing frame substitution
Decryption
Channel decoding
Comfort Noise
Channel coding
Analogue speech input
Speech encoder
Channel encoder
Digital output
274 bits per 60 msec of speech
= 4.567 kbit/s
432 bits per 60 msec of speech
= 7.2 kbit/s
Quality speech @ low bit-rate
Complete Codec - Block Diagram
4.567 kbps
TETRATETRAChannelChannelEncodingEncoding
FEC+ CRCFEC+ CRC
7.2 kbps
ImportanceFactor
Three levels of bit stream
protection FEC
analoguespeech
TXTX
TETRATETRAVoiceVoice
DecoderDecoder
RXRX
TETRATETRAVoiceVoice
EncoderEncoder
TETRATETRAChannelChannelDecodingDecoding
HiHi
Med..Med..
NoNo
4.567 kbps
8 KHz8 kHzSampling
16 bits
8 kHz8 kHzSamplingSampling
16 bits16 bits
7.2 kbps
BadFrameFlag
• How to send four speech channels down one digital pipe?
• How to “steal” 18th time slot to send synchronisation data without loosing speech?
Usual Questions
Secret of a “stolen” Frame
1 ch. encoded speech frame
60 msec
2 31 15 16 17 1
1 TETRA Multiframe - 1.02 sec
transmitted slot No.
transmitted frame No.
15 16 17 181234123412341234 1234
1817
56.67 msec
1234123412341234
18 1 2 3 1615 1
17 x 60 msec segments of encoded speech bits (1.02 sec)
1.02 sec
18th stolen frame
17 x 56.67 msec TDMA frames (0.963 sec) ÷4=241ms/ ch.
Codec selection
Speech quality
Subjective assessments
Complexity
Computational demand of speech and channel codec
Codec complexity
C = MOPS + 0.2 * RAM (kB) + 0.05 * ROM (kB)
Overall encoder complexity 11.9 MOPS
Overall decoder complexity 5.4 MOPS
Very complex Heavy processing demand
Codec performance
0
1
2
3
4
TETRA GSM
Qu
ali
ty (
MO
S)
MOS 4 Excellent quality : Imperceptible impairment
MOS 3 Good quality : Just perceptible impairment, but not
annoying
TETRA Codec performance
Factor Effect on quality
Input level change Insensitive
Frame stealing Slight degradation, not significant
Tandeming Best avoided!
Background noise at Tx Practical results impressiveVery robust
Codec performance
Quality (Q) comparison with analogue FM
0
5
10
15
20
25
30
-32 -22 -12
TETRA
FM
Quality
Q (dB)
Audio input level (dB)
TETRA
FM
Codec performance
Comparison with analogue FM
TETRATETRA
Range
Quality
Low background noise
FM
High background noise
Codec performance
ETSI demo 2 phrases Analogue2 phrases TETRA
1 Male Moderate C/N
2 Female Moderate C/N
3 Male Poor C/N
4 Female Poor C/N
TETRA
FM
Range
Quality
1 & 2 3 & 4
•TETRA provision for 4 codecs
•Enhanced codec for TETRA telephony
• AMR (Adaptive Multi-Rate)
•Provision of a new codec for military
Future enhancements
Conclusions
• Using ACELP technique, TETRA Codec provides a nearly GSM quality at almost half bit rate
• TETRA Codec provides a superior quality to FM and GSM in the high background noise environment
• Current TETRA Standard has a provision for 4 Codecs
• Additional Codec planned for TETRA Release 2 will provide even higher quality for telephony applications